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View Full Version : Ten Tec Omni C AGC improvement



KB3ZHX
Mon 13th Jul 2015, 03:16
Hello Omni C owners.. I'm sure that you know the audio derrived AGC in the Omni is not the best. It can cause some listening fatigue and cause you to have to chase the RF Gain knob. A loud signal will cause a "pop" when it enters the receiver so that the first "dit" or "dah" can blast you across the room. I've even heard the pop in very strong SSB signals when the operator keys up (especially if he is using VOX.) But then, when the AGC catches up, the signal levels out just fine.

Reading across the net I found that later Omni's had an AGC "hang time" that was controlled by a resistor and capacitor on the IF AGC board.

In using this rig for a very long time, I noticed that if strong signals were recieved within that "hang time," I could not hear the pop. But as soon as the hang time expired (noted by the s-meter dropping back down,) a strong signal would cause the pop again. This is very annoying when in a rag-chew when the sending station pauses for a moment or two, only to come back with a huge POP!

So, to the schematics I went. The manual states that this hang time is determined by R-29 and C-25. Wanting to increase the hang time, we would have to slow the discharge of C-25 so I increased R-29 from 33k to 100k Ohms... This slowed things a little, but I needed more... So I increased the capacitor C-25 from 4.7Mf to 10Mf and WOW! What a difference!

The AGC now hangs long enough to cover pauses in received CW so that the pops are eliminated.. and CW receive is so pleasant and enjoyable with this fantastic rig.

My next step is to make a board with switchable capacitor values and switch from the face of the rig. Perhaps a small rotary knob to switch between 3 different values for different situations. How high can we go with the capacitor? We'll have to test that.. But for now, the added hang time sure makes listening to the Omni much more enjoyable.. check out my Omni with other mods (N4YG Digital DDS VFO mod, AADE digital display mod, N4YG sine wave side tone mod, and W3ATT side tone spot feature) at my QRZ page..

http://qrz.com/db/W3ATT

Thanks for reading - Brian W3ATT